The Triton

Engine Room

Managing diesel engine emission gases complicated


For more than a century, the diesel engine has been the workhorse for many industries around the world powering large trucks, farm tractors, locomotives, construction and mining equipment and vessels of all types. Unfortunately, diesel engine exhaust contains pollutants in a complex mixture of emission gases and particulates, which are known to be harmful to the environment and humans.

These emissions include oxides of nitrogen (NOx), which are gases that form when diesel fuel is burned with excess air. Diesel particulate matter (PM) are microscopic particles and liquids that form during the combustion process. Gaseous compounds that result from unburned fuel and lubricating oil are called hydrocarbons (HC). And the combustion process also produces carbon monoxide (CO), a colorless, odorless gas which is very toxic.

Although harmful, diesel engines only emit a small amount of HC and CO, so engine manufacturers primarily focus on reducing NOx and PM. These last two types of emissions are inversely related, meaning a reduction in one generally causes an increase in the other. This relationship complicates their management.

In the United States, the movement towards reducing diesel emissions began in the 1970s when the federal Environmental Protection Agency (EPA) investigated engine gaseous emissions from the heavy-duty highway diesel engines. The regulation of harmful exhaust emissions by the EPA started in 1994 when the levels of NOx and PM were controlled under a “tiered” series of directives which mandated the maximum level of these airborne contaminants and the effective date of enforcement. This four tiered structure, which established progressively lower allowable emissions of NOx and PM, still governs new off-road diesel engines, including marine engines.

With each progressive step, from Tier 1 to Tier 2 to Tier 3 to Tier 4, each new generation of engines are lower in emissions and more technologically advanced than the previous. The result being that Tier 4 engines emit more than 90 percent less PM and NOx than Tier 1 engines.

For the purpose of emissions, the EPA treats marine engines differently than other off-road engines. In addition to the four tiers, the EPA categorizes marine engines based on the volume displaced by each cylinder. Category 3 engines are typically larger ones used aboard container ships, oil tankers, bulk carriers and cruise ships. Category 2 and Category 1 engines are commonly used on tugboats, fishing vessels and other commercial vessels found in and around ports. The smaller engines of the Recreation Category include “yachts, cruisers and other types of pleasure craft.”

Beginning in 1997 the International Maritime Organization (IMO), through MARPOL Annex VI, began implementing the multi-tiered system for diesel emissions. The IMO regulated PM and NOx like the EPA, but also added another pollutant called sulfur oxides (SOx) to their mandate. The amount of SOx is directly related to the amount of sulfur contained in diesel fuel.

MARPOL Annex VI not only set global requirements for vessels cruising anywhere in the world, it also designated Emission Control Areas (ECA) which have more stringent emission limits. Currently there are four of these special areas: the Baltic Sea ECA, the North Sea ECA, the North American ECA which includes most waters within 200 miles of the U.S. and Canadian coast, and the U.S. Caribbean ECA which covers the waters around Puerto Rico and the U.S. Virgin Islands.

The EPA fully harmonized its diesel emission rules with that of MARPOL Annex VI in 2009. On June 27, 2011 the EPA and the U.S. Coast Guard issued a joint letter regarding the IMO’s air pollution prevention requirements. That letter’s salutation read, “Dear Shipowners, Ship Operators, Shipbuilders, Marine Diesel Engine Manufacturers, Marine Fuel Suppliers and any other interested groups:”

The body of the letter was a reminder that “all U.S. flagged vessels, and non-U.S. flagged ships operating in U.S. waters, must comply with MARPOL Annex VI regulations. Some of the regulations are applicable to ships of 400 gross tonnage and above while other regulations are applicable to all ships.”

This letter goes on to state that both the U.S. Coast Guard and EPA may bring enforcement action for a violation, and may levy a civil penalty up to $25,000 for each violation. And every day that the violation continues may be considered a separate offense, which can mean another $25,000 fine. Given the severity of the potential penalties, yacht captains and engineers are strongly advised to conduct due diligence for their vessels by contacting the EPA or USCG compliance offices.

Capt. Jeff Werner has been in the yachting industry for almost 25 years. Contact him through and

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